The subject invention relates to spring probes for electrically accessing various parts of printed circuit boards, semiconductor devices and other electric and electronic components. More particularly, the invention relates to an improved spring probe having an elongated electrically conductive contact and an elongated helical coil disposed about the beam and attached to the contact. The spring includes an offset spring lower end configured so that, when the spring probe is compressed, the elongated contact firmly contacts the spring lower end and establishes a direct electrical path between the contact head and the spring lower end.
FIG. 1 shows a conventional spring-loaded contact probe. As shown in FIG. 1, such probes generally include a movable plunger 2, a barrel 3 having an open end 4 for containing an enlarged diameter section or bearing 6 of the plunger, and a spring 5 for biasing the travel of the plunger in the barrel. The plunger bearing 6 slidably engages the inner surface of the barrel. The enlarged bearing section is retained in the barrel by a crimp 7 near the barrel open end.
The plunger is commonly biased outwardly a selected distance by the spring and may be biased or depressed inwardly into the barrel, a selected distance, under force directed against the spring. Axial and side biasing of the plunger against the barrel prevents false opens or intermittent points of no contact between the plunger and the barrel. The plunger generally is solid and includes a head or tip for contacting electrical devices under test. The barrel may also include a tip opposite the barrel's open end.
The barrel, plunger and tip(s) form an electrical interconnect between the electrical device under test and test equipment and as such, are manufactured from an electrically conductive material. Typically the probes are fitted in cavities formed through the thickness of a test plate or socket. The test plate or socket assembly process includes placing the test probes into either precision custom-machined plastic subassemblies or injection molded subassemblies formed from costly precision injection mold tools. Generally, a contact side of the electrical device to be tested, such as an integrated circuit, is brought into pressure contact with the tips of the plungers protruding through one side of the test plate or test socket for maintaining spring pressure against the electrical device. A contact plate connected to the test equipment is brought into contact with the tips of the plungers protruding through the other side of the test plate or test socket. The test equipment transmits test signals to the contact plate from where they are transmitted through the test probe interconnects to the device being tested. After the electrical device has been tested, the pressure exerted by the spring probes is released and the device is removed from contact with the tip of each probe. In conventional systems, the pressure is released by moving the electrical device and probes away from one another, thereby allowing the plungers to be displaced outwardly away from the barrel under the force of the spring, until the enlarged-diameter bearing of the plunger engages the crimp 7 on the barrel.
The process of making a conventional spring probe involves separately producing the compression spring, the barrel and the plunger. The compression spring is wound and heat treated to produce a spring of a precise size and of a controlled spring force. The plunger is typically turned on a lathe and heat-treated. The barrels are typically deep drawn and heat-treated. All components may be subjected to a plating process to enhance conductivity. The spring probe components are assembled either manually or by an automated process.
To assemble an internal spring configuration spring probe, such as that shown in FIG. 1, the compression spring is first placed in the barrel, the plunger bearing 6 is then inserted into the barrel to compress the spring, and the barrel is roll crimped near its open end forming a crimp 7 to retain the plunger. Some internal spring configuration probes consist of two plungers each having a bearing fitted in an opposite open end of a barrel. The two plungers are biased by a spring fitted in the barrel between the bearings of each plunger.
As can be seen, the assembly of the probes and sockets is a multiple step process. The fabrication of the sub-assemblies requires costly custom machining or complex tooling. Considering that probes and sockets are produced by the thousands, a reduction in the equipment, materials and steps required to produce them can result in substantial savings.
An important aspect of testing integrated circuit boards is that they are tested under high frequencies. As such, impedance matching is required between the test equipment and integrated circuit so as to avoid attenuation of the high frequency signals. As discussed earlier, the probes are placed in cavities in a test socket. Due to the numerous probes that are used in a relatively small area in the socket, the spacing between probes is minimal, making impedance matching infeasible. In such situations, in order to avoid attenuation of the high frequency signals, the length of the electrical interconnects formed by the probes must be kept to a minimum. With current probes, when the interconnect length is minimized so is the spring length and thus, spring volume.
A spring's operating life, as well as the force applied by a spring is proportional to the wire length, the diameter of the wire forming the spring, and the diameter of the spring itself (i.e.: spring volume). These requirements for a given spring operating life and required spring force are in contrast with the short spring length requirements for avoiding the attenuation of the high frequency signals. For example, in internal spring configuration probes, the compressed or solid length of the spring is limited by the barrel length minus the length of the plunger enlarged bearing section, minus the length of the barrel between the crimp and the barrel open end and minus the distance of plunger travel. Since the diameter of the spring is limited by the diameter of the barrel which is limited by the diameter of the cavities in the test sockets, the only way to increase the spring volume for increasing the spring operating life, as well as the spring force, is to increase the overall barrel length. Doing so, however, results in a probe having an electrical interconnect of increased length resulting in the undesirable attenuation of the high frequency signals.
Typically, for a given application a given spring compliance is required. Probe spring compliance is defined by the distance of spring extension from its fully compressed position to its fully extended position in the probe. Consequently, with conventional probes the volume of the spring is limited by the required compliance. A longer spring incorporated in a conventional internal or external spring probe will reduce the plunger stroke length and thus, reduce the distance that the spring can extend from a fully compressed position. Thus, for a given probe, as the spring compliance increases, the spring volume decreases and so does the spring operating life.
An alternative type of conventional probe consists of two contact tips separated by a spring. Each contact tip is attached to a spring end. This type of probe relies on the walls of the test plate or socket cavity into which it is inserted for lateral support. The electrical path provided by this type of probe spirals down the spring wire between the two contact tips. Consequently, this probe has a relatively long electrical interconnect length which may result in attenuation of the high frequency signals when testing integrated circuits.
Thus, it is desirable to reduce the electrical interconnect length of a probe without reducing the spring volume. In addition, it is desirable to increase the spring volume without decreasing the spring compliance or increasing the electrical interconnect length. Moreover, it is desirable to provide a probe that can be easily and inexpensively manufactured and assembled.
Accordingly, it is an object of the subject invention to provide a new and improved test probe that is small enough to accommodate the increased density of leads on modern integrated circuit (IC) chips.
A further object of the subject invention is to provide a test probe that has durable and flexible contacts for connecting a component to a printed circuit board (PCB).
A further object of the subject invention is to provide a reliable test probe that will continue to operate as designed after numerous operational cycles.
Yet another objective of the subject invention is to provide a test probe that is capable of accepting leading edge IC packages such as ball grid array (“BGA”), chip scale packages (“CSP”), land grid array (“LGA”), quad fine pitch no lead (“QFN”) packages and others.
A further object of the subject invention is to provide a new and improved test probe that is inexpensive to manufacture and has a minimum number of parts.
A further object of the subject invention is to provide a test probe that does not damage the pads of the printed circuit board onto which it is mounted.
Another object of the subject invention is to provide a test probe that is suitable for use in high frequency test applications.
A still further object of the subject invention is to provide a test probe, that is operative to establish a minimum resistance, minimum inductance electrical connection between the lead of an integrated circuit and a printed circuit board.
Additional objects and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations pointed out in the appended claims.